1. Field of the Invention
The invention relates to a controllable motion damper with a damping piston, said damping piston separating two damping chambers from each other in a damping cylinder, and with a damping valve disposed in a connecting channel of the two damping chambers, the transmissivity of said damping valve for a damping fluid contained in the motion damper being controlled by a control device.
2. Description of Background Art
Such a controllable motion damper for an artificial knee joint is known from DE-OS 10214357. The damping cylinder in said motion damper is filled with a magnetorheological fluid serving as damping fluid, said magnetorheological fluid being displaced by the axial movement of a damping piston in the damping cylinder, wherein the magnetorheological fluid flows through a damping valve, said damping valve connecting the two cylinder chambers in the damping cylinder, said cylinder chambers being separated by the damping piston. Depending on the transmissivity of the damping valve, correspondingly larger or smaller forces acting on the damping piston are required in order to displace the damping piston and thus to achieve the desired damping effect. In the known motion damper, the device controlling the transmissivity of the damping valve is in the form of one or more electromagnets which, through a magnetic field controlled for higher or lower intensity, correspondingly change in known manner the viscosity of the magnetorheological fluid and thus its flow resistance in the damping cylinder. In the known publication, two illustrative embodiments are provided for the arrangement of the damping valve. According to one embodiment, the damping valve consists of a gap between the damping valve and the damping piston, the gap being permeated by a controllable magnetic field. According to a further embodiment, the two damping chambers separated by the damping piston are connected by a bypass channel in which the magnetorheological fluid flows when the damping piston moves. The bypass channel passes through a magnetic field generator, wherein the controllable magnetic field of said magnetic field generator controls the viscosity of the magnetorheological fluid flowing in the region of the magnetic field in the respectively desired manner for more or less damping.
A further known motion damper is described in EP 0957838 B1, said motion damper likewise containing a magnetorheological fluid as the damping fluid.
The magnetorheological fluid (MRF) used in the known motion dampers is a fluid which, because it contains small magnetizable particles, has a high relative density in the range of approximately 3, this therefore being close to half the relative density of iron. Consequently, in view of the required fluid charges of the motion damper, the motion damper is made very heavy, this representing a considerable disadvantage with regard to its practical use. Above all, the viscosity of an MRF cannot be increased at will, i.e. saturation occurs, which means that, in spite of higher magnetic power, it is not possible to achieve any increase in damping.
Further known from the international application WO 99/27273 is a motion damper in which, for the purpose of reducing the volume of a magnetorheological fluid in the damper, one damping chamber thereof contains a conventional damping fluid, e.g. oil, while the other damping chamber contains the magnetorheological fluid, which, however, additionally acts as the damping fluid, thereby accounting for a considerable proportion of the weight of the motion damper.
Also known are motion dampers for artificial knee joints in which only a conventional damping fluid, especially oil, is used (e.g. U.S. Pat. No. 6,113,642) and in which the control of the throughflow of the damping fluid is accomplished by means of electromagnetically operated slide valves in the form of damping valves, the operation of which requires considerable electrical currents, this necessitating a correspondingly strong power supply for the damper.
Such motion dampers for artificial legs are controlled by programs which are stored in electronic controllers. An example thereof is contained in GB 2328160 A.